The invention relates to a powder-feeding system for gas-propelled delivery of a powder to be controllably supplied, as to a flame-spraying gas torch or to a transferred-arc or non-transferred plasma torch.
Various schemes have been proposed for the aspiration of powder into a flow of carrier gas, but for one reason or another they have lacked the precision with which it is desirable to controllably meter the delivery of powder to a torch for torch deposition to a surface to be coated or otherwise treated by the powder.
U.S. Pat. No. 3,976,332 attacks the problem by so configuring a downwardly open orifice in a carrier-gas tube within the convergent lower region of a powder-supply hopper that, in the presence of a fluidizing-gas flow from the top of the hopper and through the body of powder in the hopper, a fluidized powder regime will be established in the immediate vicinity of the orifice, thereby providing fluidized powder under pressure for conveyance by the carrier-gas flow. This technique has the disadvantage that the pressure drop of fluidizing gas must necessarily be some function of the instantaneous head of undischarged powder within the hopper.
U.S. Pat. Nos. 4,381,898 and 4,391,860 operate on the principle of fluidizing powder in a fluidizing chamber which receives fluidizing-gas flow from below a porous screen, in the context of a gravitational flow of powder via a relatively small central discharge location above the porous screen. A carrier-gas conduit extends transversely through the fluidizing chamber; this conduit has an upwardly facing opening facing the powder-discharge location, and a shed between this opening and the powder-discharge location sufficiently laps the opening that only gas-fluidized powder can be delivered by carrier-gas flow.
Precisely controlled and satisfactory as the latter system has proven for conveyance of fluidized powder in a single line of carrier-gas flow, there have been problems in the delivery of powder at the torch, resulting in a lack of uniformity, i.e., in an inability to control uniform torch deposition of powder, particularly for the greater rates of deposition which the powder feeder is able to deliver.
Attempts to solve the problem by splitting powder (and carrier-gas) flow into two separate delivery hose lines, in order to have powder enter a torch flame, particularly a plasma flame, have been unsuccessful at a satisfactory level of flow rate. And when rates are adopted to achieve desired quantities of powder delivery, the flame (particularly in the case of a plasma flame) becomes deflected and a substantial fraction of the powder is wasted by not penetrating the flame sufficiently for proper melting to occur.